LANDSCAPE ECOLOGY A Top-Down Approach - Chapter 7 pps

Harris and James Sanderson External Impact Theory Application of the Theory Habitat Conservation Plan Re-Membering Fragmented Landscapes Restoring Landscape Processes: the Case for Movem

Part III Landscape Theory

and Practice

The Re-Membered Landscape

Larry D Harris and James Sanderson

External Impact Theory

Application of the Theory

Habitat Conservation Plan

Re-Membering Fragmented Landscapes

Restoring Landscape Processes: the Case for Movement

Corridors

Wolf Reintroduction

Wolves Require Management

Migration Corridor Identification

Putting Things Together: An Ecology of Landscapes

Example

How did the composite set of ecological processes get so out of balance so as

to produce the deranged, dysfunctional, dismembered landscapes we have today? We cannot summarize the course of human history here Humans have proven to be nearly infinitely adaptable and to accept the present as the way things have always been That is, changes made by humans occur so rap-

idly that they become the status quo in short order For instance, how long has

the Glen Canyon Dam blocked the waters flowing into the Grand Canyon? For much of the U.S population the answer is “the dam has always been there.”

A complacent acceptance has quieted what should be outrage We are pointed not to be able to see walruses along the Northwest Atlantic shores from Cape Cod to Greenland We desire their return The problem is not that walrus populations cannot be restored; the problem is that widespread ignorance of

disap-the previous existence of walruses prevails History, and not just ancient tory, is being forgotten.

his-Humans have now developed technology sufficient to alter nearly all cesses affecting landscapes—deliberately and otherwise There are proposals

pro-to use nuclear weapons pro-to deflect potential earth impacpro-tors long before they are even close to the planet Humans have altered the chemical composition

of the atmosphere, warming the global climate Can we now alter the ocean currents with such changes? Humans realized the value of movement corri-dors as the need for communication and trade increased The U.S Highway Act created the most extensive and complex movement corridors on earth for the benefit of humans With few exceptions this has proved disastrous for ecological processes, especially wildlife movement River courses have been altered and repeated attempts have been made to control the flow of the great rivers of the world including the Yangtze River in China The disastrous neg-ative effects on huge delta ecosystems such as the Mississippi delta and the Nile delta have become appreciated and apparently been relegated to the dustbin of history The complete alteration of the regional climate surround-ing the Aral Sea in Asia and the nearly total degradation of land- and sea-scape processes was achieved by humans in less than 30 years Cattle have overgrazed the western U.S from Mexico to Canada, at a cost that can scarcely be calculated and shouldered by the American public for the benefit

of a comparatively few citizens

These changes in and of themselves are remarkable achievements over, they are cumulative, rarely canceling previous alterations Recognition

More-of the negative effects More-of these changes and numerous others wrought by humans has lead to a few restoration efforts However, restoring nature has proved elusive

The U.S Department of Agriculture estimates that restoration and creation projects have added more than 400,000 ha of fresh- and saltwater wetlands in the U.S since 1982 In 1998, the Clinton Administration called for the creation

of another 80,000 ha of new wetlands each year for the next decade The goals

of the project were that the wetlands should be “functionally equivalent” to undisturbed, natural wetlands But can nature be recreated? Experiments are underway now that compare recreated wetlands and their nearby natural systems (Malakoff 1998)

Landscape fragmentation continues around the world, creating yet more dismembered landscapes Once landscapes are dismembered other ecologi-cal processes such as invasions of “weedy species” occur, further changing natural ecological processes Recently, Wahlberg et al (1996) created a model

to predict the occurrence of endangered species in fragmented landscapes The modeling approach was presumed to be a practical tool for the study and conservation of species in highly fragmented landscapes In the model, the probability of local extinction was determined by the size of the habitat patch Isolation from occupied patches and the size of the patches determined the probability of colonization of an empty patch Empirical data to support the

model came from studies of the Glanville fritillary butterfly (Melitaea cinxia)

The model was then used to predict the patch occupancy of the false heath

fritillary butterfly (M diamina) The benefits of such a model are numerous

Can such a model be useful for all species?

The size and isolation of the patch were used to determine the presence of butterflies in patches From our landscape perspective the analysis on the contextual setting of each habitat patch is equally as important as the patch itself That is, if an isolated habitat patch was considered close to occupied patches by some distance metric, then the isolated path would, with high probability, be occupied Linear distance, however, is a poor metric to mea-sure isolation To appreciate this, suppose, for instance, that all isolated patches within 100 m of each other were occupied and that one isolated frag-ment was separated by a mere 50 m and a six-lane superhighway from these occupied patches The model would predict that the isolated patch would be

occupied without regard to the physical barrier created by the highway over, between-patch physical distance was assumed to be invariant for all

More-species This suggests that a bald eagle would have just as much difficulty as

a mouse in attempting to occupy the habitat patch, presuming both occupied nearby favorable patches

Isolation of favorable patches can be enhanced by the content of the vorable patches (Merriam 1991) An otherwise favorable fragment might lay surrounded by a city as in the case of Central Park in New York Nearby noise

unfa-or light pollution might adversely affect birds munfa-ore than rodents, enabling the latter to colonize patches that no bird would enter, however close a favor-able patch might be Therefore, we must conclude that linear distance is an inadequate currency to measure the colonization ability of a species because different physical barriers to colonization are species dependent The dis-tance measurement must, at a minimum, be modified to be a “degree of dif-ficulty” measurement that varies between species A contextual analysis is critical to understanding species distributions Deciding when a patch is small enough or isolated enough, or determining how wide a corridor must

be to enable species movement is not the answer to re-membering mented landscapes

frag-The General frag-Theory of Insular Biogeography

With the previous examples in mind and other well-understood situations

we now state four fundamental theories of landscape ecology: Edge Theory, Juxtaposition Theory, External Impact theory, and Corridor Theory Using these theories we can create a General Theory of Insular Biogeography Note that these theories do not depend on the size of the fragment, reserve, pro-tected area, or hot spot

own data on Oncifelis guigna, a small forest cat, supported and extended this

conclusion My data suggested that male carnivores were more likely to fer human-caused conflicts than females This was because males had home ranges that overlapped several female home ranges Male ranges most often included human homes, and males traveled between females and therefore invariably came into contact with humans, their pets, and domestic fowl Males more frequently crossed roads, thus risking exposure to domestic dogs Inevitably, males were more tempted to take domestic fowl, especially free-ranging fowl, than females

suf-Edges and patches also affect the quality of movement corridors We know that edges invite invasive species and that nearby unfavorable habitat nega-tively influences corridors Would a panther use a linear forest path bisecting

a university campus, for instance? The theories we have presented can be applied to the analysis of landscape connectivity and patch influence

Juxtaposition Theory

Processes within landscape fragments are affected by processes acting in proximate fragments The impact of the effect extends beyond the boundary

of the fragment and depends upon the strength of the process

Juxtaposition Theory says that processes such as human activities affect other processes acting within fragments For instance, night light pollution negatively impacts birds in otherwise suitable habitat Nearby noise or light pollution is a proximate process

Corridor Theory

Corridors increase population persistence in fragmented landscapes

Fahrig and Merriam (1985) and Merriam (1991) discussed the role corridors

in patchy habitats played in the demographics of small rodents There were three demographic effects of interpatch dispersal First, interpatch movement enhanced metapopulation survival Second, interpatch dispersal supplemented population growth in certain instances Third, patches where extinction occurred were recolonized The greater the connectivity between patches, the more likely the metapopulation was likely to persist Merriam (1991) concluded

that connectivity was critical to species long-term survival But what constitutes connectivity?

Species-specific behavior determines whether or not suitable corridors and landscape connectivity exist Merriam (1991) noted that the assessment of con-nectivity must therefore come from species-specific empirical studies That is, looking at a highly detailed vegetation cover map and quantifying habitat is simply not good enough to determine if landscape connectivity exists for the mobile species considered Movement behavior must be known

External Impact Theory

Processes within landscape fragments are affected by external processes whose origin, time of arrival, and strength of impact cannot be known in advance Nevertheless, with certainty an external process will severely negatively impact natural functioning processes within the landscape fragment

A hurricane is a natural process that acts episodically During hurricane son, the probability of an isolated fragment of beach being hit by a hurricane is near zero However, we can say with total certainty that eventually the isolated beach will be hit The probability of complete destruction is probably again small; however, given enough time, disaster will occur Hurricanes, acid rains,

sea-or metesea-orite impacts are examples of processes acting on fragments that are not

of proximate origin That is, these processes originate elsewhere and then travel stochastically, impacting fragments in their path

These four theories are supported by many examples and have been mental research programs of several researchers Recall that the Theory of Island Biogeography as developed applied to continental islands Our four the-ories have been applied to habitat islands or patches in an often not so benign matrix These four theories lead to a General Theory of Insular Biogeograpy that makes a special case of the Theory of Island Biogeography Edge, Juxtapo-sition, and Corridor Theories do not apply to islands; however, the External Impact Theory does apply Many of the results of island biogeography apply to isolated continental fragments However, whereas negative edge effects are now widely accepted as occurring in continental fragments, edge effects were not originally part of the Theory of Island Biogeography We neither think of islands as being connected by corridors, nor juxtaposed with altered habitats

funda-We should no longer rely on the crutch of the Theory of Island Biogeography to explain results that are only remotely similar to continental islands

Application of the Theory

Assume there exists a metacommunity of species S1 and species S2 in five ent landform cover types, C1 to C5 Generally, species use cover types differently

differ-We use the word habitat to refer to those cover types acceptable (in a broad sense) to a particular species S1 and S2 utilize C1 to C5 differently according to

Table 7.1 The collection of all cover types is referred to as the universe Assume that a square or hexagonal grid overlays the universe and that each of 100 grid cells each contains a single cover type Suppose that S1 and S2 occupy different amounts of each cover type and densities vary between these types according

to Table 7.1 S1 might be humans and S2 wolves Each perceives C1 to C5 ently

differ-Different species utilize cover types differently (see Table 7.1) Optimal habitat is prime habitat for a species Suboptimal habitat is habitat that is less than optimal habitat, perhaps where reproductive and foraging success are high, but not optimal Marginal habitat refers to habitat where the species can survive, but might not adequately reproduce Invasible habitat is habitat not currently unoccupied, but could be if conditions change Habitat that is not traversable acts as a barrier to dispersal and movement to the species and remains unoccupied All but nontraversable habitat is assumed to be travers-able, thus the number of traversable habitat cells is the sum of the number of optimal, suboptimal, marginal, and invasible cells

To compute average habitat quality for each species, habitats must have an associated value We assume that each species values each cover type differ-

ently First, we compute the total population of each species

The sums run across all cover types because the habitat for a particular cies varies with the cover type In general, the total population of Sj in i dif-ferent habitats is given by:

Number per cell (2)

Average habitat quality over the region for Sj can be calculated by ing values to each habitat Let optimal habitat have a value of 8, suboptimal habitat a value of 6, marginal a value of 4, and invasible 2 Nontraversable habitat has a value of 0 Note that C1 above is optimal habitat for S1 and so has a value of 8 while simultaneously has a value of 6 for S2 because the hab-itat is suboptimal for S2 Let vi,j be the weighting assigned to habitat i for Sj.

assign-For instance,

For S2,

Overall, the area occupied by S2 is of lower quality because of the large number of suboptimal habitat cells Habitat quality can be weighted by the population residing in the habitat:

We find

and

Q2 > Q1 because a higher percentage of the total population of S2 occupies higher quality habitat than does the total population of S1

Habitat connectivity can be measured as the fraction of the universe

occu-pied by traversable cells If the grid is regular (rectangular, hexagonal) we can

then assign a probability that a corridor exists through the universe using the results from the Percolation Theory Note that habitat connectivity depends not on cover type, but on the habitat type and is thus dependent on the par-ticular species For Sj, habitat connectivity, HC, is:

HCj = (number of cells in universe - )/(number of cells in universe)

where the ith cover type is nontraversable habitat Hence

where the ith cover type is nontraversable habitat

Note that habitat fragmentation when added to habitat connectivity sums

to unity:

HFi + HCi = 1Often a landscape appears to have suitable cover types, but the organism

of particular interest is not present Although trite, things are not always what they appear to be We can slice, dice, and categorize landscape features and cover types (Gustafson 1998) However, we prefer to provide an example

of landscape contextual analysis Figure 7.1 shows a hypothetical landscapeoverlaid with 100 hexagonal cells Each grid cell is assigned a habitat value for a particular organism At first appearance, the landscape appears to have many favorable cells, and one might conclude that populations of the partic-ular organism of interest would be healthy The classification is similar to that used above; however, we have adapted it for a contextual analysis as follows Our contextual analysis will be based on a set of rules depending on the

“sphere of influence” that different cover types have on a particular ism The organism-specific rules will be applied in order For the hypothetical organism used here, detrimental cells have a sphere of influence greater than the space they occupy For example, sound from these detrimental cells might

organ-HC1=(100−C1i) 100=(65 100)= 0 65

HC2=(100−HC1) 100= 0 45

HFi H numbers of cells in the universe

i 1

travel across the landscape and impact the particular organism negatively For other organisms, this sound might have no influence and so the sphere of influence of the detrimental cells would be less To account for this influence, all neighboring cells will be changed to marginal from whatever classifica-tion they were assigned.

cells will be assigned as marginal Marginal habitat also has a sphere of influence beyond its border

hab-itat will be assigned suboptimal Thus, detrimental cells affect not

FIGURE 7.1

A fragmented landscape of 100 hexagonal cells Empty cells are optimal

habitat, light gray are suboptimal, darker gray are marginal, and black are

detrimental.

Color Cover type % of landscape

Light gray Suboptimal 22

only their immediate neighbors, but also their once-removed neighbors.

neighbors to suboptimal Suboptimal cells have edge effects that are damaging to optimal cells

neigh-boring optimal cells

The result of applying a contextual analysis to the hypothetical landscape

in Figure 7.1 yields Figure 7.2 with:

Although the landscape in Figure 7.1 appeared to have many optimal and suboptimal cells, the contextual influence of marginal and detrimental habi-tat and edges effects considerably reduced the number of these habitats.Furthermore, the influence of detrimental cover types often extends differ-entially in one or more directions, or can leapfrog across a landscape such as happens when fire in sugar cane fields carries nutrients deep into the south-ern Everglades In this case, the influence of detrimental cells extends 100 km

or more during particular seasons Negative edge effects also reduce able favorable habitat (Figure 7.2)

avail-Obviously, more complex rules can be applied to the contextual analysis of landscapes These rules can be empirically derived in some cases Contextual analysis enables an analytic exploration of landscapes beyond content and appearance In the case of the Florida Everglades, detrimental areas sur-rounding the national park have a large sphere of influence that can now be quantitatively studied Contextual analysis can be applied to study the migration route of the monarch butterfly, for instance, because we can extend the analysis of content across the landscape based upon a set of rules derived from theory that are species specific

Habitat Conservation Plan

Section 10 of the Endangered Species Act of 1973 was amended in 1981 to include that each designation of a threatened or endangered species required the creation of a habitat conservation plan (HCP) An HCP is a written docu-ment that specifies how much land must be set aside to protect threatened

Color Cover type

% of landscape

Light gray Suboptimal 51